This invention relates to a method for testing the connection of a liquid crystal display and thin film transistor (the connection of both hereinafter called TFT-LCD) that is used as an array and color LCDS employing TFTs.
Recently active-matrix LCDS are most widely used as color liquid crystal displays. This type of LCDS is characterized by a design that permits writing necessary signals into a liquid crystal only for a given short period of time. At other times, the gate of the input circuit leading to the liquid crystal is kept open to hold the written signals. Thus, the liquid crystal functions like a dynamic memory.
The gate is thus closed only for a limited period of time required for the writing of necessary signals and opened at other times. This is why thin-film transistors (TFT's) or field-effect transistors (FETs) are commonly used.
The use of TFTs-LCDS as active-matrix LCDS for color displays has been increasingly recently.
FIG. 1 shows the outline of a TFT-LCD, whereas FIG. 2 is a circuit diagram indicating the interconnections of the components in a TFT-LCD. Because a sheet of liquid-crystal display contains many picture elements, extremely large numbers of TFTs 4, gate lines 3 and data lines 5 are used in correspondence therewith.
However, not all TFTs always function properly. Not all connections between TFTs 4 and data lines 5 and gate lines 3 and between individual gate lines and data lines are always properly made, too.
Before completing liquid-crystal displays, therefore, it is essential to determine that they are in proper condition.
With no efficient method to conduct such test established, however, conventional TFT-LCD systems have been assembled without making such preliminary test. Otherwise, each individual TFT has been checked for proper wiring with their peripheral devices by using cumbersome practices.
To solve this problem, the applicant proposed a new technology in the method and apparatus for testing TFT-LCDs in Japanese Patent Application No. 133796 of 1993.
This technology tests the wiring condition of TFT-LCDs in an active color LCD in which the drain of a TFT is connected to a grounded cell capacitor Cs. As shown in FIG. 3(a) the cell capacitor of an LCD is charged through a data line by turning on the TFT for a given period of time. Specifically, when a gate voltage V.sub.G and a data voltage V.sub.D are applied to the circuit shown in FIG. 2 and the switch S.sub.d is turned on, the cell capacitor C.sub.s is charged with the data voltage V.sub.D at the time that a pulse voltage of the gate voltage V.sub.G exists. Next, the charged condition is maintained by turning off the TFT for a given length of time. Specifically, after the pulse of the gate voltage V.sub.G is removed and the TFT is turned off, the resistance between the source and drain of the TFT becomes 10.sup.5 to 10.sup.6 times greater than that built up while the TFT is turned on by the gate voltage V.sub.G. Therefore, the cell capacitor C.sub.s remains charged, with the charge held thereby decreasing only slowly through leakage. Then, the TFT is turned on again to release the stored electric charge through a resistor connected to the ground through the source and drain of the TFT. Specifically, when the TFT is turned on again between the source and drain thereof with the passage of a pulse of the gate voltage V.sub.G, the charge held in the cell capacitor C.sub.s is discharged through the resistor R.sub.g connected to the source of TFT. Whether or not the wiring is proper is determined by checking the waveform of current or voltage induced by the discharge.
To achieve the check of the waveform of current or voltage described above, the conditions of discharge with maximum and minimum current or voltage through a properly functioning and wired TFT-LCD are predetermined as shown in FIG. 5. Then, whether the actual discharge current or voltage is within the pre-established range between the maximum and minimum limits can be determined.
In this test, however, the discharge output itself does not does not arise from the output of the cell capacitor C.sub.s because the test is conducted under the influence of the stray capacitance between the TFT and data line and between the data and gate lines.
When such a stray capacitance is present, a wrong wiring could be mistaken for a proper one.
The chance of such misjudgement is high especially when small-capacity cell capacitance C.sub.s is used to increase the percentage aperture of liquid crystals.
The object of this invention is to provide a method for testing TFT-LCDs that assures correct judgement of the connection of TFT-LCDs and operation of TFTs even in the presence of the stray capacitance mentioned before.